Intercessory prayers for health or healing are requests to an object of worship for the preservation or restoration of health. There has been a recent proliferation of clinical trials that compare the health outcome of a group of prayed-for patients with that of controls, to test the efficacy of intercessory prayer. In this essay, the author defines the concept of intercessory prayer, contrasts it with other forms of prayer, and reviews the literature concerning clinical trials of its efficacy. The arguments put forward in favor of conducting such trials and those against are described and the reader is invited to consider their relative merits. The author concludes by discussing the potential power of faith in healing, reviewing the philosophical basis and pitfalls of clinical trials of intercessory prayer, and urging readers to weigh the arguments for and against such trials in academic medicine.